This invention relates to wide bandgap materials, particularly for solar cells.
In the case of solar cells, photosensitive material such as silicon is deposited on or associated with a transparent conductive oxide. In many cases there can be an undesirable interaction between the silicon and the transparent conductive oxide. A common transparent conductive oxide is tin oxide which, with respect to silicon, is thermodynamically unstable. In particular, the free energy of formation for tin oxide is about -124 kilocalories per mole at standard temperature and pressure (STP), while the free energy for formation of silicon dioxide at STP is about -192 kilocalories per mole. The rate of interaction between the silicon and the transparent conductive oxide depends upon temperature. At elevated temperatures (above about 300 degrees C. to 400 degrees C.), the rate of interaction is fast and, in the case of tin oxide and silicon, leads to a chemical reduction of the transparent conductive layer. There is also the oxidation of the silicon layer in the vicinity of the tin oxide layer and the diffusion of elemental tin into the silicon layer. As a result the transparent conductive oxide loses its transparency, as well as its conductivity. An insulating silicon oxide layer forms and tin diffuses throughout much of the silicon layer.
For an amorphous silicon solar cell having the structure glass/tin oxide/p-i-n/metal, where the p, i and n denotes p-type, intrinsic, and n-type hydrogenated amorphous silicon, respectively, it is desirable for the p-type layer to have a wide optical bandgap in order that most of the incident light can be absorbed in the photovoltaically active i-layer of the device. Such wide gap doped layers are practically non-absorbing for visible light and are called window layers. The bandgap of a semiconductor is defined as the energy gap between the top of the valence band and the bottom of the conduction band.
A common technique for the formation of semiconductive materials involves the use of plasma deposition or plasma-assisted chemical vapor deposition. Unfortunately, these techniques involve both "energetic" particles and atomic hydrogen which may increase the degree of interaction between the semiconductor layer and associated transparent conductive layers. For example, in the plasma-assisted chemical vapor deposition of hydrogenated amorphous silicon (a-Si:H) on tin oxide, tin has been observed well into the silicon layer using elemental depth profiling techniques (
Appl. Phys. Lett. 43, 101-1983).
The bandgap of intrinsic (undoped) a-Si:H is typically 1.75 eV. When a-Si:H is doped p-type the bandgap usually shrinks to about 1.4 eV and the material becomes strongly absorbing. For this reason the source materials for the doped semiconductive p-type layer often include carbon in an attempt to widen the bandgap. Unfortunately, the presence of carbon in intrinsic (i) layers of hydrogenated amorphous silicon has been reported to cause impairment in the quality of the layer (J. Non-Cryst. Solids, 66, 243-1984). Accordingly, although carbon is commonly employed in wide bandgap materials, it may be desirable in many situations to achieve such bandgaps without the employment of cabon--thus reducing the possibility of subsequent carbon contamination of the i-layer.
Accordingly, it is an object of the invention to facilitate and improve the production and utilization of wide bandgap materials. For wide gap hydrogenated amorphous silicon, the bandgap should exceed about 1.9 eV. A related object is to achieve wide bandgap materials which avoid many of the disadvantages of the prior art.
Another object of the invention is to realize wide bandgap materials which can be employed in conjunction with other associated materials without cross contamination. A related object is to increase the compatibility between transparent conductive oxides, used, for example, in the fabrication of solar cells, and associated semiconducting layers. Another related object is to increase the compatibility between tin oxide and an associated silicon material used in the manufacture of solar cells.
Still another object of the invention is to achieve wide bandgap materials without the creation of excessively energetic particles such as ions or electrons. An associated object is to provide alternative techniques for the realization of wide bandgap materials without the employment of plasma or plasma-assisted deposition. A closely related object is to achieve a wide bandgap p-layer at the low temperature (180 degrees. C.-300 degrees C.) used in plasma-assisted deposition but without the plasma or energetic ions or electrons.
Still another object of the invention is to obtain wide bandgap materials which are practically transparent to the desired optical energy. A related object is to obtain wide bandgap materials with and without carbon alloying.